The guanine cap of human guanylate-binding protein 1 is responsible for dimerization and self-activation of GTP hydrolysis.

Human guanylate-binding protein 1 (hGBP1) belongs to the superfamily of large, dynamin-related GTPases. The expression of hGBP1 is induced by stimulation with interferons (mainly interferon-γ), and it plays a role in different cellular responses to inflammatory cytokines, e.g. pathogen defence, control of proliferation, and angiogenesis. Although other members of the dynamin superfamily show a diversity of cellular functions, they share a common GTPase mechanism that relies on nucleotide-controlled oligomerization and self-activation of the GTPase. Previous structural studies on hGBP1 have suggested a mechanism of GTPase and GDPase activity that, as a critical step, involves dimerization of the large GTP-binding domains. In this study, we show that the guanine cap of hGBP1 is the key structural element responsible for dimerization, and is thereby essential for self-activation of the GTPase activity. Studies of concentration-dependent GTP hydrolysis showed that mutations of residues in the guanine cap, in particular Arg240 and Arg244, resulted in higher dissociation constants of the dimer, whereas the maximum hydrolytic activity was largely unaffected. Additionally, we identified an intramolecular polar contact (Lys62-Asp255) whose mutation leads to a loss of self-activation capability and controlled oligomer formation. We suggest that this contact structurally couples the guanine cap to the switch regions of the GTPase, translating the structural changes that occur upon nucleotide binding to a change in oligomerization and self-activation.

Biochemical properties of the human guanylate binding protein 5 and a tumor-specific truncated splice variant.

The human guanylate binding protein 5 (hGBP5) belongs to the family of interferon-gamma-inducible large GTPases, which are well known for their high induction by pro-inflammatory cytokines. The cellular role of this protein family is unclear at this point, but there are indications for antiviral and antibacterial activity of hGBP1. hGBP5 exists in three splice variants, forming two different proteins, of which the tumor-specific one is C-terminally truncated by 97 amino acids, and therefore lacks the CaaX motif for geranylgeranylation. Here we present biochemical data on the splice variants of hGBP5. We show that, unlike hGBP1, hGBP5a/b and hGBP5ta do not bind GMP or produce any GMP during hydrolysis despite the fact the residues involved in GMP production from hGBP1 are conserved in hGBP5. Hydrolysis of GTP is concentration-dependent and shows weak self-activation. Thermodynamic studies showed strongly negative entropic changes during nucleotide binding, which re fl ect structural ordering in the protein during nucleotide binding. These structural changes were also observed during changes in the oligomerization state. We observed only a minor in fluence of the C-terminal truncation on hydrolysis, nucleotide binding and oligomerization of hGBP5. Based on these similarities we speculate that the missing C-terminal part, which also carries the geranylgeranylation motif, is the reason for the dysregulation of hGBP5's function in lymphoma cells.

Structure and function of the FeoB G-domain from Methanococcus jannaschii.

FeoB in bacteria and archaea is involved in the uptake of ferrous iron (Fe(2+)), an important cofactor in biological electron transfer and catalysis. Unlike any other known prokaryotic membrane protein, FeoB contains a GTP-binding domain at its N-terminus. We determined high-resolution X-ray structures of the FeoB G-domain from Methanococcus jannaschii with and without bound GDP or Mg(2+)-GppNHp. The G-domain forms the same dimer in all three structures, with the nucleotide-binding pockets at the dimer interface, as in the ATP-binding domain of ABC transporters. The G-domain follows the typical fold of nucleotide-binding proteins, with a beta-strand inserted in switch I that becomes partially disordered upon GTP binding. Switch II does not contact the nucleotide directly and does not change its conformation in response to the bound nucleotide. Release of the nucleotide causes a rearrangement of loop L6, which we identified as the G5 region of FeoB. Together with the C-terminal helix, this loop may transmit the information about the nucleotide-bound state from the G-domain to the transmembrane region of FeoB.

Catalytic core of a membrane-associated eukaryotic polyphosphate polymerase.

Polyphosphate (polyP) occurs ubiquitously in cells, but its functions are poorly understood and its synthesis has only been characterized in bacteria. Using x-ray crystallography, we identified a eukaryotic polyphosphate polymerase within the membrane-integral vacuolar transporter chaperone (VTC) complex. A 2.6 angstrom crystal structure of the catalytic domain grown in the presence of adenosine triphosphate (ATP) reveals polyP winding through a tunnel-shaped pocket. Nucleotide- and phosphate-bound structures suggest that the enzyme functions by metal-assisted cleavage of the ATP gamma-phosphate, which is then in-line transferred to an acceptor phosphate to form polyP chains. Mutational analysis of the transmembrane domain indicates that VTC may integrate cytoplasmic polymer synthesis with polyP membrane translocation. Identification of the polyP-synthesizing enzyme opens the way to determine the functions of polyP in lower eukaryotes.